Back. I used to be a prolific taxi passenger and I've only been in the front a few times in a cab full of other people. On a single occasion several years ago I sat in the front at the driver's invitation.
Front but i guess it's a cultural thing. Here in Germany you usually have both options without most drivers getting worked up over it (i've never even had so much as a strange look when i sat down in front). I've also never seen a german taxi with bulletproof glass between the front and back and mandatory back seating. These drivers sure do get attacked, injured etc but apparently not so much that the companies or the government feel the need to establish a safe zone for the driver.
Front or back works for me, although if the taxi is one of those dressed-up civilian cars then I will usually sit in front. If the vehicle is a mini-bus type of taxi, I'll usually sit in the back even though the option to ride in front is usually still there. The only exception is the black Hackney carriage where the driver will only usually take you in the back. So to speak.